1. Field of the Invention
The present invention generally relates to devices for testing communications equipment, and more particularly to a device for testing the operation of an optical fiber which may be carrying an active signal.
2. Description of the Prior Art
In recent years, fiber optic cables have replaced traditional copper wire as the preferred medium for telecommunications. Although optical fibers have certain advantages over copper wire, they are still subject to faults (which may result during installation or from environmental factors after installation), and may also accidentally be "miswired," i.e., improperly routed to or from the optical transmitters and receivers. For these and other reasons, it is often necessary to identify one particular fiber out of a large group of such fibers carried in a common cable.
Identification of a particular optical fiber is an inherently difficult task, since direct connection of the fiber to an optical detector would require cutting the fiber, which is highly undesirable. First of all, the fibers being tested are often active, and cutting the fiber would lead to an enormous data loss. Also, a cut fiber must be reconnected by the use of an optical splice which attenuates the communication signals and can create interfering reflections. Furthermore, since a trial-and-error method must often be used to pinpoint the particular fiber, this could result in literally dozens of cut fibers and splice assemblies. Color-coding the outer buffer of the fibers is insufficient where a large number of fibers are present, or where the records of the color-coding are incorrect. Fortunately, a technique has been devised for detecting test signals being carried on an optical fiber without the need of cutting the fiber.
This technique relates to the manner in which a portion of the transmitted light "leaks" out at microbends in the fiber. The degree to which the fiber must be bent in order to obtain such leakage is a function of the relative indices of refraction between the fiber core and its cladding, and between the cladding and the outer buffer. In U.S. Pat. Nos. 4,728,169 and 4,790,617 (both issued to Campbell et al.), this principle is used to optimize the alignment of two fibers which are to be spliced together. Those patents describe a manner of injecting light into the fiber as well as withdrawing a portion of the light.
In U.S. Pat. No. 4,759,605 issued to Shen et al., this same principle is used to extract light from a fiber without cutting the fiber. The device disclosed in that patent utilizes a convex element to forcibly urge the fiber into contact with a concave surface. The concave surface is light transmissive and is attached to an optical detector. Shen et al. does not give any indication of the mechanical means by which the convex element is moved relative to the concave surface.
A more detailed description of an optical fiber handling apparatus is given in European patent application No. 89300290.7 filed by James et al. (related devices are shown in European Patent Application No. 89300330.1, and Patent Cooperation Treaty Application No. PCT/GB88/00225). That apparatus utilizes a spring-loaded plunger which forces the fiber against a concave waveguide. The disclosed embodiments, however, suffer from several disadvantages. First of all, the leakage occurs along a 180.degree. arc, which creates excessive loss. This could lead to fatal attenuation of traffic comprising actual communications data. The structure of the James et al. device also requires two other 90.degree. curves in the fiber, resulting in further losses.
Additionally, the spring loading of the plunger creates an abrupt bending action which is undesirable for two reasons. First of all, this may result in mechanical damage to the fiber. Secondly, it can disrupt transmission of data traffic in the fiber. Relaxation (i.e., release) of the fiber can also disrupt transmission if it occurs too quickly. This is due to the fact that optical amplifiers will ramp the amplification up or down depending upon the strength of the transmitted signal, in order to achieve a more stable output. Quick bending or unbending of the fiber creates a momentary change in the signal strength which may be shorter in duration than the response time of the optical amplifier, leading to unacceptable amplifier performance. This is particularly crucial for high-speed data transmission, such as DS3 communications which transmit about 45 megabits per second.
Finally, there is no mechanism in the James et al. apparatus for limiting the force applied by the plunger; even if the plunger were moved in a controlled fashion, excess stationary force could result in deformation of the fiber, which might require repair by cutting away the damaged portion and splicing the fiber back together. It would, therefore, be desirable and advantageous to devise an apparatus for identifying optical fibers which minimizes disruptions in real data traffic, and avoids excess deformation of the fiber under test.